Spatial Division Multiple Access (SDMA) is a method of multiplexing several signal streams, each one targeted to a different destination, simultaneously, by utilizing multiple transmit antennas. An SDMA channel access method may enable the use of the same frequency at the same time to communicate with several stations located in different places. For example, an SDMA Access Point (AP) having multiple antennas may use a beamforming technique to transmit to several remote stations simultaneously. Each transmit antenna may transmit the intended signal multiplied by a certain weight, and by dynamically controlling the weights of each antenna the transmission may be directed to a desired location. Under certain assumptions, it can be shown that data transmissions to N user antennas can be multiplexed together using at least N transmit antennas
In Multi-User Multiple-Input-Multiple-Output (MU MIMO) communication systems, a multi-antenna transmitter may transmit simultaneously to several multiple-antenna receivers, using SDMA technique. Typically, the problem of designing the TX beamforming vectors, which are typically the beamforming vectors of the AP, is considerably simpler if the total number of receiver antennas, summed over all receivers, is not larger than the number of transmitter antennas.
Designing TX beamforming vectors may require full or partial knowledge of Channel State Information (CSI) of the channel. In explicit sounding exchange, the AP may send a sounding frame from which the intended beamformees may estimate CSI. In sounding feedback, each beamformee can return CSI. This sounding exchange may impose a considerable overhead that may reduce goodput and useful bandwidth of the communication system.